Gun safety proposals also succeeded in California and Washington state, where voters approved measures that seek to keep guns and ammunition out of the hands of violent people.

Under federal law, licensed gun dealers must run background checks, but for people who sell at gun shows and over the internet, background checks are optional. According to public opinion surveys, more than 80 percent of Americans favor so-called “universal background checks,” but federal legislation has languished in Congress. Most recently, in June, the U.S. Senate rejected a bill in a 44-56 vote.

In the absence of a national law, eight states -- California, Colorado, Connecticut, Delaware, New York, Oregon, Rhode Island and Washington state -- had enacted universal background checks before the 2016 election. In five of those states, Democratic governors signed background-check legislation in the wake of an elementary school shooting in Newtown, Conn., that left 26 adults and children dead.

In Maine and Nevada, both state's Republican governors opposed universal background checks. But voters broke from their leadership in Nevada, where the measure passed with a margin of less than 1 percent.

Two years ago, Washington state made history by becoming the nation's first state to enact new gun regulations by popular vote. Given that success and the passage of more voter-approved proposals this year, gun control ballot measures could become more common.

“In states where lawmakers have not moved fast enough, or at all, we definitely think this is going to be a good model,” said Ari Freilich, a staff attorney at the San Francisco-based Law Center to Prevent Gun Violence, which endorsed the California and Washington state ballot measures.

Besides the proposals in Maine and Nevada, ballot measures in California and Washington state added further restrictions on guns and ammunition.

California:Proposition 63

The proposition before California voters was an extensive package of reforms, but the provision that received the most attention requires people to pass background checks before they purchase ammunition.

The measure also bans ammunition magazines capable of storing more than 10 rounds, establishes criminal penalties for gun owners who fail to report a lost or stolen firearm and sets up a process for removing guns from residents who have recently been convicted of felonies or other serious crimes.

Nearly 63 percent of voters approved the measure. The proposition had some high-profile endorsements from Lt. Gov. Gavin Newsom and both of the state's U.S. senators, but it also drew criticism from groups representing local law enforcement.

In a public statement, Ken Corney, president of the California Police Chiefs Association, said the proposition “fails to meet the appropriate balance between public safety and individual gun rights.” One provision, for example, could have the unintended consequence of prohibiting police recruits from training with large-capacity ammunition magazines.

Corney said his group preferred a bill passed by the state legislature earlier in the year, partially because it would be easier to amend legislation passed by lawmakers than by voters. Getting a super majority of lawmakers to agree to an exemption for cadets would be “almost unattainable when dealing with an issue as controversial as firearms regulation,” he wrote.

Washington State: Initiative 1491

In Washington state, voters decided to empower courts to temporarily take away firearms from people considered to be at high risk of harming themselves or others -- a tactic only one other state uses.

Under the measure, law enforcement, family members and housemates can petition the courts to prevent someone who poses a violent threat from possessing or purchasing a gun for up to one year.

California was the only state that already had a law like that, but the general idea borrows from the more widespread practice of prohibiting convicted domestic violence abusers from buying or possessing firearms. Legislators in at least 11 states plan to introduce bills next year that will be similar to what Washington's voters approved.

Although the idea is popular among gun-control advocates, the Law Center to Prevent Gun Violence has criticized the California law for having a design flaw: Officers are not required to make sure that people actually relinquish their guns. Instead, California has relied on an honor system in the hopes that people would voluntarily comply with the court orders.

The Washington state version addresses that enforcement problem and require officers to provide a record of surrendered firearms to the courts and the gun owner. If it appears that the person didn't surrender his firearms, the court can issue a search warrant for the missing guns.